1. Field of the Invention
This invention relates to a combustion state-determining system for internal combustion engines, for detecting a misfire occurring therein, to thereby determine a state of combustion thereof, based on the misfiring rate of the engine, and also to a combustion state control system for controlling the combustion state of the engine, based on the determination result of the combustion state-determining system.
2. Prior Art
There are conventionally known various kinds of combustion state-determining systems for internal combustion engines, as proposed, for example, by Japanese Provisional Patent Publication (Kokai) No. 4-81548 by the present assignee, which finally determines that the engine is in a misfiring state when misfires have occurred in the engine at a predetermined rate or more, and then alerts the driver to take an appropriate failsafe action to prevent component parts of the exhaust system, especially a catalytic converter for purifying exhaust gas components, of the engine from being adversely affected by misfires.
In the above proposed conventional system, the predetermined misfiring rate is set based on a result of tests conducted beforehand, to such a value above which degradation in the performance of exhaust system component parts, especially the catalytic converter, is accelerated. Therefore, when the performance of the engine is degraded or the performance of the catalytic converter is deteriorated due to aging, the predetermined misfiring rate becomes unsuitable for determination of a misfiring state. As a result, it can happen that the engine is not determined to be in a misfiring state even when misfires have occurred at a high misfiring rate which causes accelerated deterioration of the catalytic converter.
Further, in the proposed conventional system, it is immediately determined that the engine is in a misfiring state which may accelerate deterioration of the catalytic converter when the misfiring rate of the engine exceeds a predetermined reference value (i.e. predetermined misfiring rate). In actuality, however, the temperature of the catalytic converter elevates to a level which accelerates deterioration of the catalytic converter, only after the lapse of a certain time period from the time the misfiring rate exceeds the predetermined reference value. In other words, the engine is not always in a combustion state which accelerates deterioration of the catalytic converter, even when the misfiring rate exceeds the predetermined reference value.
Furthermore, when the engine is finally determined to be in a misfiring state, a warning is issued to alert the driver to take an appropriate failsafe action, or when the engine is finally determined to be in a misfiring state which accelerates deterioration of the catalytic converter, some control is carried out to restrain deterioration of component parts of the exhaust system. However, the failsafe and control operations are not always perfect to properly restrain deterioration of exhaust system component parts.